Method of magnetic investigation



Jan. 20, 1953 o. H. SCHMITT METHOD OF' MAGNETIC INVESTIGATION Filed Aug. '7, 1944 Ill- @9h Quand Patented Jan. 20, 1953 UNITED STATES i@ TEN T V` '-2 Claims. il

inventionrelatesto methods magnetic investigation 'and more particularly -to methods for vrinvestigatin'g ltlfie felds -=of '-magnetic -bodies under laboratory conditions.

It is often desirable in studies 'of :magnetic bodies torconduct laboratory investigations 'of fthe magnetic elds `produced thereby. For -this purpose ftliebo'dy to `rlbe investigated may Yitse'ltbe set up -in'- the 'laboratory A4or, 'if -it is l'arge vor unwieldy, the body 'may z'be reproduced f'by ^a scale model. It 4is"wel'lflm'ovvn thatin'ma-gne'tic studies a'magneticfbody-may 15e-replacedbyascale model if `the dimensions"of Ithe model andthe distribution 'o'f magnetization thereof are Vfaithfully reproduc"edatthe reducedV scale. AUnder these conditions,"the -magnetic Afield 'producedby the -model will, at al'l"points, b'eara -scale `relation to 'that of'the'actualbo'dy. Ineith'er case, themagnetic field "produced `may be measured "using conventional instruments, as JJfor "example magnetometers. AIn n'a 'training device "in which `a -submerged submarine is `simulated `for the 'purpose of tracking said simulation With magnetic field sensitive instruments, theknown'lmagne'tic characteristics of Avarious types fof submersible J'craft are "reproduced either in scaled models, `or as fin the' jp're'sent case, 'by the inagneti'cal equivalents.

Although 'investigations have been carried vvkout using the methods referred to above, diniculties are usually encountered dueto interference by stray steady "magnetic "fields p'revffilent in tire usual electrical ".laboratory. While the eects of such Astray fields m'ay be `largely 'eliminated by magnetic shielding or through' the' use `'orneutraliz'ing `nel'da .such "procedures *are .cumbersome and require .the use cf special equipment'not'always available. .'Anotheriprocedure .involves amatliematical analysis of .the measured .field Yof the body taking .into account'the effects o'f'the stray fields, .but this lis tedious and depends 'for its accuracy .upon the .exact .determination di the stray fields.

l 'Previous 4laboratory methods `have "the :further disadvantage that, when 'thejmagne'tization or the "body is `due principally nto induced effects causedby tl'feeartlisun'agnetic field, investigation is vlimited tothe `'eld produced vby the body-at the'particularlatitude 'of the Vlaboratory "since ythe direction of the earths magneti-cieldy'and hence the induced effects, vary with the'ge'ographical location.

In `additionfthe instruments used Hfor 'accurate measurement o'f the steady magnetic Yiiields produced'byamagnetic body are complicatedfan'd require 'careful Yoperating techniques. Their use,

therefore, maybe fdisadvantageous ifa large num# ber vofi'eld determinati-ons are-"torbemada Likewise, .such instruments are :often arran'ge'd to measure 'the total field 'and Yconsidera'ble culty .is'fenc'ounteredrin .ineasuringthe component of the iield of rthe :body tto :'beinves'tigated 'ein a particular direction.

:Itlisian object of theinventionptherefore, provide :a method `fornivestigation of the field of a magnetic ib'o'dy ivhichr-mayzbe'used '.intne laboratory "or lsi'mde'r. similar conditions such fthat the 'feld to be 'measured :is .considerably :less A:in intensity than :the straylfieldeiisting.atttlrefisame location.

It is 'ian'other bj ect 'fof .t'hezinventio'n ,.to'zpro'vi'de a 'magnetic iield .'investigation method whereby the component fof the :magnetic field-iof :a body any 'particular direction `|may ibe measured cdila rectly.

It is Ta further robje'ct `of .-.thelinventi'on ltc yapro" Vide a magnetic eld investigation method which may be :carried out `u'singcs'izrilole rte'cnniques fand relatively .simple .measuring instruments.

AIn :view .fo-f the :.above, lthe invention fprdvidesA thef-coilarrayattlie chosen point, determining-fa calibration factor 'tor the lmeasuring device ftrom the data .thus obtained, Jand Tthereafter using 'tithe calibrated measuring "devi'cfe to l"Ziletermine "the field of the coi-l arrayat ltheo'ther points.

The above -an'd other Creatures of 'fthe inventi'On lWill loe "described "in the r`folloltfrlg detailed.

specic'ation 'and ip'o'intefout in the V4anper-ided claims.

inthe drawing, tne-si'nglegure is A`a circuit diagram oflappara'tus'suitableftor use practiciingth'e method ffo'f fthe inventions `-In the 1eranticaof meiner-,noa eff'theiinventien,

it is assumed that'fthe magnetization 'ofthe body to be investigated'iis'knowii orfrn-aylbefdeteriliiined.. In the event that itne-ibodyfhasibeen reproduced by a scalelrno'del,'lthermagnetizationlofftheaetual body may be lus'ed since linagnetizati'on iis-*ex-v pressed .in terms I of l-magne'tic "moment per unit volume. A coilarra'y lis 4then-1arrangedto ireizla't-:e

" the magnetic vbody for the purpose lof producing the magnetic eld to be investigated. It is well known that the magnetic eld produced by a body having any given magnetization may also be produced by means of a current distribution determinable from the given magnetization. In general, the desired current distribution may be obtained through the use of a suitable coil array which may comprise a single coil or several coils, the turns of which are distributed to produce the required current distribution. In some cases, the proper configuration of turns may be determined analytically; while in others, an experimental determination is suitable.

When the proper coil configuration is excited with direct current, a steady magnetic field scaled to that of the body to be investigated is produced. If, on the other hand, alternating current excitation is used, the eld produced will be identical in direction to that of the body at every point but Will have a sinusoidally varying magnitude. Thus and according to the invention, alternating current from a suitable source is used to excite the coil array, a convenient current amplitude being chosen arbitrarily. When a detection device such as a simple pickup coil is placed in the alternating magnetic field, an alternating E. M. F. is generated therein, the R. M. S., average, or peak amplitude of Which may conveniently be measured by suitable means, as for example a vacuum-tube voltmeter.

According to the above, alternating-current excitation is supplied to the coil array to produce a desired current distribution. The amplitude of such excitation may be chosen arbitrarily, since it affects only the magnitude of the field produced by the array. The amplitude of the excitation to the coil array having been chosen arbitrarily, the measuring device may be calibrated as described below to read directly the strength of the eld at any point in relation to the coil array.

The viield produced by the body under investigation is ordinarily known or may be computed at some chosen point. The E. M. F. produced in the pickup coil by the magnetic field of the coil array simulating the body under investigation is measured at a corresponding point in relation tor the coil array. The excitation to the coil may be varied arbitrarily if necessary to bring the E. M. F. to a convenient value for measurement. 'I'he E. M. F. so determined corresponds to a eld strength at the chosen point and may arbitrarily be set equal to the measured field strength of the actual body to be investigated at the chosen point. Then, if field strengths are to be measured in gammas and E. M. VF.s in volts, a calibration factor in terms of gammas per volt may be obtained. After the detection device has thus been calibrated, field strengths may be read directly at any point. Thus, so long as the coil array produces a field the intensity of which at all points varies linearly with the excitation current through the array, the measuring device may be used to read directly the eld at any point even though the current distribution or the magni-l tude of excitation current is varied. It is therefore possible, for example, to increase excitation when measuring the field at remote points and.V

thus increase accuracy of measurement.

lInasmuch as the stray magnetic elds found in the usual electrical laboratory are essentially steady in character, they will have substantially no effect on a measuring device of the pickup-` coil type so long as the measuring device is sta-- tonary ,in respect to the eld. Thus, the only field measured by the pickup coil under these conditions is the alternating eld produced by the coil array.

If the method of the invention is to be used for dynamic investigations, that is, for continuous determinations of the field strength as the measuring device is moved relatively to the eld, the comparatively small E. M. F.s generated in the measuring device as the pickup coil travels through the stray fields will be of relatively low frequency and may easily be discriminated against by means of simple selective circuits incorporated in the measuring device.

Since the earths magnetic field is essentially a steady field, it will have no elect on the measuring device, the output of which will depend only upon the alternating field of the coil array and will be the same irrespective of the direction of the earths field. Thus, the eld of the body at any geographical location may be simulated at the laboratory merely by altering the current distribution of the coil array suitably to compensate for changes in the induced magnetization of the body which would occur if it were actually moved t0 a new location.

In the above, the measuring device has been referred to as being of the pickup-coil variety. Conveniently, it may comprise a simple pickup coil either with or without a core of magnetic material and a Vacuum-tube voltmeter arranged to read either R. M. S. or peak values of theA E. M. F. generated in the pickup coil. Since` only the pickup coil of the measuring device need be moved about in making field measurements, it may easily be oriented in any desired direction in space and when so oriented will measure only the component of the field of the.

body in the chosen direction. Determination of the field component in any directionv is, then,

merely a matter of orienting the pickup coil with its axis in that direction. Resolution of the field in the chosen direction then occurs automatically.

Since the magnetic field produced by a given magnetic lbody at a particular point may be of f either polarity depending upon the magnetiza-l tion of the body, it is necessary to simulate' polarity of field when the alternating-currentexcited coil array is substituted for the body toF be investigated. This may conveniently be done by arbitrarily letting the alternating eld produced when the excitation source is connected, directly to the coil array represent a steady magnetic field of one polarity. A steady eld of opposite polarity is then simulated by shifting thel phase of the coil excitation through and j letting the out-of-phase field generated represent` the required steady eld. The phase reversalV rayl oscilloscope to compare the phase of the I E. M. F. generated in the pickup coil with that of a reference signal derived directly from the source of excitation.

In some cases, the coil array required to produce the necessary current distribution may beunduly complicated and di'icult to construct as a single coil. It Ymay be desirable, therefore, to,

use a plurality of coils in place of a complicated single coil to produce theV desired Vcurrentdistribut'ion. "Such "a procedure @is particularly a'dvantageous when the magnetization ofthe body is such that the eldproduced'thereby -may l:be considered Ato ybe "due Yto a body having three resultant magneticmoments which add vectorially to produce "atotalmagneti'c moment. Thus, three coils having :distributed fturns, i'as .reuuired, may conveniently be used to repreducerthe mag'- netic field of the body, these coils being used respectively'to reproduce the 'elddue to each of thecomponent moments. 1n this case, :the current excitation to .the .three l"coils must :be properly phased .in Yorder .to .preve'nt generation of la gyra'ting .eld vector.

@ne :system f or use in .accordance with the method of the invention .is shown in the :single figur-cof thedrawing. In 'thiscasa it is .assumed that the current distribution may be produced by three mutually .perpendicular coils I8, ':I.2Sfand I4, each havingsuitably distributed turns. shown in the drawing, coil I0 "is disposed vertically, coil "I2 vhorizontally,*and coil I4 fperp'endicularly to l-eachp'f coils ltr-and I2. 'These coils will be referred `to 'hereafter as' thevertical, -horizontal and transverse coils, respectively. Excitation for the three component coils is provided by a single oscillator I6, and the frequency of excitation is chosen to minimize interference from the alternating-current power lines of the laboratory in which the field measurements are to be made.

The output of oscillator I6 is applied to the input terminals of each of three similar amplifiers I8, and 22, potentiometers 24, 28 and 28 being provided to vary the signal supplied to the amplifiers. Ampliers I8, 20 and 22 may be standard audio-frequency amplifiers having constant phase shift and means, knobs I8', 28', and 22 respectively for phase adjustment. One of these amplifiers is associated with each of the three component coils, amplier I8 being arranged to furnish excitation to vertical coil IIJ, amplifier 20 to horizontal coil I2, and amplier 22 to transverse coil I4. Double-pole, doublethrow switches, 30, 32 and 34, respectively, are included in the circuits between amplifiers I8, 20 and 22 and the associated coils so that the phase of the excitation current to any of the component coils may be reversed. Thus, the output terminals of each amplifier are connected to the blade terminals of the associated switch, the xed terminals of which are arranged as in the usual reversing switch. The component coils are connected to one set of the xed terminals in each case.

In each case a known fixed resistor 38 is included in the circuit connecting the amplifiers to the blade terminals of the switches. Provision is made through the use of a double-pole, threeposition switch 38 for connecting a vacuum-tube voltmeter 40 across any one of xed resistors 36, thereby to permit measurement of the excitation current supplied to each of the component coils.

The eld measuring device comprises a simple pickup coil 42 shown in the drawing as having a core of magnetic material, although this is not necessary, and a vacuum-tube voltmeter 44 arranged to measure the E. M. F. generated in the pickup coil. Conveniently, vacuum-tube voltmeter 44 may be arranged to read the R. M. S. value of the E. M. F. generated in the pickup coil, although other types of voltmeters may also be used. A cathode-ray oscilloscope 46 is provided for determining the polarity of the alternating eld produced by the coil array. One set of dethe use 'of -eselector switch f 4-8,lbe connected "either across ypickup coil 42 oracro'ss the inputtermirra-ls lof vacuum-tube voltmeter 48. The 1 other set of v deflecting plates Iof vthe 4oscilloscope vis r`connected Vto the 'output terminals Afof oscillator v48. Thus, it will be'un'derstood that the oscilloscope may y`-be-used to '-compareth'e phase 'oi'l'the excitation at the output Vof the foscillator with fthat furnished 'by ampliiiers I8, -20 i'and 22 -to l'their respective componerrtlcoils *I8, "I2 'fand I4, for Ato compare the phase of "thev li/l. '-F. generated pickup coil 42 with that of the excitation trom oscillator "I'.

lIn the use *of the system-described above, vthe proper distributionfoi turns Vfor `each/df*the-cornpcnen'tcoils lIll, -"|2 and y'I4 to reproducethefld of'th'e'body to "be vinvestigatedis determined from the magnetization of the`bcdy. The oscillator is then se't for any convenient loutput and *the excitation currents supplied 'to "each rof the "coils are thenproperly phasedbycomparing the phase of the-current/to each coilin turn Ywith thato'f theo'scillator output -using the cathode-'ray 'oscillos'cope. For rthis purpose "selector switch L48 v'is used in the position shown by the dashed lines. In accordance with one convenient procedure, the phase of the excitation current to each coil is adjusted to have the same value in respect to that of the oscillator output, thus insuring that the three coils are excited in phase. Potentiometers 24, 26 and 28 are then used to adjust the relative amplitudes of the excitation currents supplied to the three coils to produce the desired current distribution and the amplitude of excitation in each case is determined by using vacuum-tube voltmeter 40, which is connected successively by selector switch 38 across fixed resistors 36, associated with the vertical, longitudinal and transverse coils, respectively.

Each of reversing switches 38, 32 and 34 is then arbitrarily set in one of the two possible positions. Pickup coil 42 is then oriented in space until it is parallel to each of component coils IIJ, I2 and I4 in turn, and the phase of the E.. M. F. generated in the coil is in each case compared with that of the oscillator output using the cathode-ray oscilloscope with selector switch 48 in the position shown in solid lines. Since the field polarity due to a resultant moment in a given direction may be determined from the geometry of the system, the phase relationship corresponding to that polarity may be found. Reversing switches 30, 32 and 34 are labeled accordingly and may be used thereafter to vary the current distribution in the coil array to simulate various magnetizations of the body under investigation.

Since, from the geometry of the system, the polarity of the resultant field due to the body having a total magnetic moment of known direction can be determined, the phase relationship between the output of pickup coil 42 and the output of oscillator I corresponding to that polarity may be found. There after the polarity of any field due to the body under investigation may be determined by comparing the phases of the two outputs used in this original determination. Vacuum-tube voltmeter 44 may then be calibrated to read eld strengths directly according to the method already described above.

What is claimed is:

1. The method of investigation of the field of a magnetic body comprising the steps of substituting for said body a vertical coil, a horizontal coil, and a transverse coil each having an axis perpendicular to the others, energizing said coils With alternating currents of known amplitude and frequency, positioning aV pickup coil at a point in the fields of said coils magnetically corresponding to a point of known magnetization of said body, measuring the voltage induced in said pickup coil by the fields of said other coils which is proportional to the known magnetic intensity at said point, moving said pickup coil to other positions in the eld of said other coils, and measuring voltages at said other points thereby determining the magnetization pattern of said body.

2. 'I'he method of investigation of the eld of a magnetic body of known magnetization comprising substituting an array of coils having interceptingields for said body, energizing said coils with alternating currents of known frequency and amplitude, positioning a pickup coil Vat a point in the eld of said coils corresponding in field strength Vand polarity with a known point of magnetization in the field of said body, measuring the voltage induced in said pickup coil at said point, moving said pickup coil through the eld of said array of coils, and measuring the voltages at other Ypoints in said eld thereby;4

The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,297,929 Taylor Mar. 18, 1919 1,461,492 Moody July 10, 1923 1,672,328 Loth June 5, 1928 1,819,798 Shimizu Aug. 18, 1931 2,149,717 Pearson Mar. 7, 1939 2,266,358 De Lanty Dec. 16, 1941 2,412,617 Jenkins Dec. 17, 1946 2,428,155 Guyod Sept. 30, 1947 OTHER REFERENCES Heiland: Geophysical Exploration; Prentice- Hall, Inc., New York; 1940; pages 19, 402-404. Weiller: Electronics, vol. 15, May 1942, pages 

